1. Field of the Invention
This invention relates generally to the field of rigid disc drive data storage devices, and more particularly, but not by way of limitation, to a method and apparatus for reducing the generation of acoustic noise in a disc drive data storage device.
2. Brief Description of the Prior Art
Disc drive data storage devices of the type known as "Winchester" disc drives are well known in the industry. In such machines, digital data are recorded on and retrieved from a thin layer of magnetizable material on the surface of spinning discs. The recording and retrieval of data--also referred to as "writing" and "reading", respectively--is accomplished using a transducer carried in a slider body which includes a self-acting hydrodynamic air bearing which "flies" the transducer a very small distance above the surface of the disc. This slider/transducer subassembly is sometimes referred to collectively as a head, and typically a single head is associated with each disc surface. The heads are selectively moved, under control of electronic circuitry, to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. In the current generation of rigid disc drive products, the most commonly used type of actuator is the rotary moving coil actuator. The discs themselves are typically mounted in a "stack" on the hub structure of a brushless DC spindle motor, and the rotational speed of the spindle motor is precisely controlled by motor drive circuitry which controls both the timing and power of commutation signals directed to the stator windings of the motor.
The first Winchester disc drives to be produced were large cabinet models which included discs having a diameter of fourteen inches and AC induction spindle motors. These types of disc drive were commonly located in dedicated "computer rooms" with large main frame types of computers, where such environmental factors as temperature and humidity could be carefully controlled. In this type of environment, the acoustic noise generated by cooling fans and disc drive motors was of little concern, since the only persons directly in contact with the systems were maintenance and operations personnel, who were generally not in the computer room for extended periods of time. The users of such systems were typically located at a remote location and communicated with the computer system via keyboards and display terminals which did not generate excessive amounts of acoustic noise.
With the advent of the personal computer, however, it is most common for a computer system to be located within the work space of the system user, which has prompted increased awareness on the part of system manufacturers of the problem of acoustic noise. Indeed, in certain markets, such as Europe, the amount of acoustic noise allowable in the work place is closely regulated and controlled. With this in mind, it has become commonplace for system manufacturers to impose a "noise budget" on manufacturers of major system components, such as disc drives, which limits the amount of acoustic noise that such components can contribute to the over-all noise of the system.
One of the principal sources of acoustic noise in disc drive data storage devices is the spindle motor which mounts the discs and spins them at a constant high speed. Typical spindle motor speeds have been in the range of 3600 RPM and in products of the current technology have increased to 4800 RPM, 7200 RPM and beyond. Analysis of various types of disc drives have brought to light several different modes of acoustic noise generation which are attributable to the spindle motor and its control logic.
An analysis of acoustic noise in one disc drive product revealed that the mode of noise generation was sympathetic vibration of the disc drive housing in response to the rotating mass of the spindle motor. When the motor was spinning at its intended 3600 RPM operational speed, the housing adjacent the spindle motor shaft vibrated at 960 Hz. In this product, the spindle motor stator was rigidly mounted to a ball bearing housing, which, in turn, was rigidly attached to the disc drive housing.
In another disc drive--with a spindle motor operating at 7200 RPM--it was found that a major mode of acoustic noise generation was the excitation of the stator mass by the application and removal of the commutation pulses used to drive the motor and control its speed. The vibration of the stator was rigidly coupled to the housing to induce an unacceptable amount of acoustic noise at a frequency of 5760 Hz. This product utilized a stationary-shaft spindle motor, and the stator was rigidly attached to the stationary shaft, which was in turn fixedly attached to the disc drive housing. The commutation pulses are timed, polarization-selected DC current pulses which are directed to sequentially selected stator windings. The rapid rise and fall times of these pulses act as a striking force, much like the contact of a hammer blow, and set up sympathetic vibrations in the stator structure. Since the stator in this motor was rigidly connected through the stationary shaft to the disc drive housing, these vibrations were close-coupled to the housing and generated resonant vibrations in the housing which were the source of unacceptable acoustic noise.
Prior art attempts to reduce or eliminate this noise have generally been confined to controlling the resonant frequency of the housing, and, in some cases, damping the vibration of the housing.
The general intent of the present invention is to uncouple the stator of the spindle motor from hard contact with the housing of the disc drive brought about by rigid attachment of the stator to the stationary portion of the motor, and thus minimize the acoustic noise caused by these modes of noise generation.
It has been found that a single inventive improvement in spindle motor design was capable of alleviating both of these situations, and it is this invention which will be discussed below.